Specific Surface Area of Barrier Mixtures at Various Outgas Temperatures

Specific surface area (SSA) values for Fe–smectite alone and mixtures comprising varying proportions of glacial till were determined by nitrogen-adsorption Brunauer–Emmett–Teller analysis under different outgassing temperatures. Additionally, the extent to which the SSA of a mixture was related to the SSA of its components was investigated. A theoretical value for mixture SSA can be computed by summing the products of component SSA and the corresponding weight fraction. This theoretical estimate may not always yield the same value as measured because of a variety of physical and chemical interactions that can lead to either aggregation or dispersion of the constituent matrix. The results are explained in terms of these interactions and within the context of SSA relevance to barrier performance. For mixtures comprising 90% glacial till and 10% Fe–smectite, measured specific surface areas were consistently higher than predicted values for outgassing temperatures beyond 100°C, with a maximum departure of 4 m2/g. An increase in the net repulsive force between particles is likely responsible for the observed change. A striking feature noted in the microporosity analysis is the collapse of the Fe–smectite interlayer at an outgassing temperature of 200°C, possibly as a result of iron reduction.